The present invention relates in general to optical modulation systems and components therefor, and is particularly directed to a new and improved apparatus and method for providing active compensation for transient thermal effects in acousto-optic modulator devices.
Acousto-optic subsystems are commonly employed in optical modulation systems to process high frequency signals. Because of the small physical volume of such subsystems and the typically random data-pattern nature of the signals they process, acousto-optic signal processing arrangements are subject to thermal transients, which can lead to distortion of the optical beam path, and degrade system performance. This thermal distortion problem is illustrated in FIG. 1, which is a diagrammatic cross-sectional side view of a prior art acousto-optic modulator.
As shown therein, a generally solid rectangular bulk acousto-optic medium 10 (such as quartz glass) is supported within a heat sink mounting structure comprised of a thermal extraction platform 11, on which a main substrate 12 is mounted. An elastomer mount 13 is inserted into a cavity 14 of the main substrate 12 and is sized to receive the acousto-optic medium 10. An acoustic energy launch transducer 15 is coupled to a top surface 16 of the bulk material 10. A thermally conductive layer cover or bar 17 overlies and is mounted flush with the top of the structure, so as to provide a rigid, thermal extraction enclosure.
As indicated by thermal flow arrows 18, the acousto-optic architecture of FIG. 1 is designed to xe2x80x98removexe2x80x99 heat through the top, bottom and sidewalls of the bulk material 10. In so doing, however, the apparatus of FIG. 1 creates a significant gradient in the thermal energy flow profile or characteristic within the bulk material. Moreover, the relatively random nature of the modulation signals applied to the launch transducer introduces a time variation to the thermal characteristic. Both of these variations can contribute to unwanted deviations in the optical beam path and thereby degrade optical performance.
Indeed, it can be expected that a multi-channel acousto-optic signal processor will be subjected to substantial variations in drive signals and thereby significant variations in thermal gradients in the vicinity of each acoustic launch electrode, as a result of the random nature of each channel of modulation signals being processed. In a typical thirty-two channel signal processor, these differing thermal conditions can contribute to substantial distortion in the optical beam path.
In accordance with the present invention, the above-described thermal distortion problem of conventional acousto-optic signal processing architectures is substantially mitigated by augmenting the electrode structure through which acoustic energy is launched into the bulk material with one or more adjacent strips of electro-thermal (resistive) material. These additional resistive strips serve to provide active and dynamic thermal compensation for transient thermal effects in both axial and side directions through the bulk material. In a multichannel system, the invention employs an interleaved distribution of electro-thermal compensation resistive strip electrodes, which are terminated in a common conductor layer.
Each auxiliary resistive strip electrode is driven by a prescribed DC bias voltage, that serves to actively introduce thermal energy into a respective surface portion of the bulk material, which is immediately adjacent to that portion of the bulk material upon which a launch transducer is mounted. This active injection of thermal energy along opposite sides of the launch transducers effectively xe2x80x98softensxe2x80x99 or xe2x80x98spreads outxe2x80x99 the thermal gradients produced thereby and thereby effectively creates a predictable thermal profile within the bulk material.
In order to effectively xe2x80x98flattenxe2x80x99 the overall or composite thermal gradient characteristic of a multi- (e.g., thirty-two) channel system, each channel has a dedicated a control circuit for establishing the appropriate electrical bias voltage to be applied to a respective resistive strip. A respective control circuit includes a comparator having has a first input coupled to receive a first voltage representative of a prescribed level of electrical energy to be applied to the electro-thermal element. A second comparator input is coupled to an RF power detector that monitors the RF power being applied to the launch electrode adjacent to the resistive strip. A third input of the comparator is coupled to a DC power detector, which monitors the DC power supplied to the resistive strip by the output of the comparator.
The voltage applied to the first input of the comparator is set at a prescribed value that is higher than the average RF power that will be imparted to the launch transducer. If a voltage representative of the detected RF power is less than this voltage, the comparator will actively drive its output with a voltage corresponding to the difference between the prescribed level and the detected RF value, as the comparator monitors the DC power detector to determine how much additional power, if any, is to be dynamically applied to the resistive strip. This active injection of thermal energy into the bulk material serves to establish an a priori thermal gradient characteristic across all the channels of the signal processor, and dynamically compensate each channel on an individual basis, so as to enable the invention to effectively track and compensate for time-dependent variations in heating, resulting in a substantially stable thermal profile. With the bulk material having a stable thermal profile, optical beam path distortion is no longer an issue.